An RBS spokesman apologised for the inconvenience caused and said debit card payments were now being processed as normal. But at 1240 GMT a number of customers were still complaining on the social media site.

"Is there a problem now?" one customer, Andrew Gibson, asked in a tweet to RBS. "My card was declined via contactless and chip and pin."

Others said the issue had embarrassed them at check out lines or left them unable to buy lunch.

The RBS spokesman reiterated that the issue was fully resolved. "No customers will be left out of pocket," he added.

The fault comes after an IT outage at mid-sized British lender TSB, owned by Spain's Sabadell, plunged the bank into crisis and brought into sharp focus the consequences for customers of lenders' technology failures.

(Reporting by Emma Rumney; Editing by Alexandra Hudson)